The objective of this research is to obtain an understanding of the role of lipids, especially the phospholipid polar head groups, in membrane structure and function. The main approach to be used in this research will be to alter the lipid composition with respect to the fatty acids, the polar head groups and the sterol content of Lm cells in tissue culture. Subcellular membrane fractions, including the plasma membrane, endoplasmic reticulum and mitochondria, will be isolated from the cells with different lipid compositions. The lipid composition of isolated membranes will also be altered by enzymatic and lipid-fusion techniques in order to compare them to the properties of membranes altered in vivo. The changes in the lipid composition will be correlated with changes in the physical and biochemical properties of the membranes. The physical properties of the membranes will be studied by a number of spectroscopic methods: fluorescence, circular dichroism, electron spin resonance, and deuterium magnetic resonance. A variety of lipid-requiring enzymes will be studied to determine the effect of altering the lipid composition on their activity. This will include two enzyme systems, adenyl cyclase and the microsomal electron transport system, that may require lateral diffusion in the membrane for their function. Experiments will be directed at understanding the importance of an asymmetric distribution of phospholipids across the bilayer. These studies should provide unique information on the contribution of lipids to the activity of membranous enzymes and the significance of the lipid composition for the fluidity and structure of mammalian cell membranes.